A screw conveyor is a device that uses a rotating helical screw blade, usually within a housing, to move material in several places in the cellulose industry, based on the principles of the Archimedean screw. It is also known as e.g. an auger conveyor, spiral conveyor or worm conveyor.
A screw feeder is very similar to a screw conveyor in its basic structure, but also compresses the material. The screw feeder comprises a conically shaped housing with a screw having a continuous screw thread for advancing the material through the housing while simultaneously compressing the material. Screw feeders of this kind are used in the cellulose industry in places where an air-tight material plug is desired for rendering it possible to feed the material against a high pressure (e.g. liquid column and gas pressure). Such screw feeders, thus, are often used for feeding lignocellulose containing material into an impregnation vessel or digester. The screw feeder may also dewater the material.
Today there is a problem that the screw conveyors and screw feeders are not operating completely filled. Historically, control of the screw has been made by controlling the rotational speed or the axial speed of the screw. The result is, however, in practise, that the screw will operate less filled the higher the rotational speed or axial speed is. Thus, the production capacity is not increased as wished by the increase in rotational speed or axial speed.